In recent years, with spread of the Internet, the traffic amount of a trunk communication system increases rapidly. In particular, the traffic amount in a link connecting networks constructing the trunk communication system (herein below, described as “interlink”) is enormous. Therefore, to continuously provide stable communication, a high-reliable technique of suppressing occurrence of congestion caused by concentration of traffic and avoiding interruption of communication due to abnormality such as disconnection of an interlink, trouble in any of nodes at both ends of the interlink (herein below, called “interlink connection nodes”), and the like is extremely important.
In the following, as an example of a network realizing a high-reliable trunk communication system, a network system (herein below, called “RPR network”) to which RPR (Resilient Packet Ring) disclosed in non-patent document 1 is applied will be taken. A technique of improving reliability of an interlink connecting a plurality of RPR networks will be described. The non-patent document 1 is a standardized document issued from IEEE (the Institute of Electrical and Electronics Engineers) in 2004.
FIG. 19 is a diagram illustrating an example of the RPR network. The RPR is a network protocol for transferring a frame (packet) on a network of a ring topology as shown in FIG. 19.
In a communication system as shown in FIG. 19, the RPR network is constructed by eight nodes that operate in conformity with the RPR (herein below, described as “RPR nodes”), and one terminal is accommodated under command of each of the RPR nodes. The expression “a terminal is accommodated under command of an RPR node” denotes that, to the RPR node, a terminal which does not belong to a ring network to which the RPR node belongs is connected. A terminal may belong to a ring network other than the ring network to which the RPR node belongs or may belong to the other communication network. A terminal accommodated under command of an RPR node will be also simply described as a terminal under command.
In the example of FIG. 19, each of RPR nodes 800 to 870 has ports P1 to P3. Each of the RPR nodes 800 to 870 transmits/receives an RPR frame to/from a neighboring RPR node by using the ports P1 and P2. Each of terminals 1300 to 1370 has a port P1. Each of the RPR nodes and a terminal connected to the RRP node transmit/receive a frame (Ethernet frame) by using the port P3 of the RPR node and the port P1 of the terminal. For information, Ethernet is a registered trademark.
As a main feature of the RPR, a high-speed protection function is widely known. For example, in the case where a link between RPR nodes is disconnected in an RPR network, immediately after the RPR nodes on both sides of the link detect the disconnection, a message of the disconnection is promptly notified to all of the other RPR nodes. The other RPR nodes which have received the notification of occurrence of the failure shift to an operation state of transmitting traffic so as to bypass the link disconnected part. As a result, the communication can be performed continuously.
The RPR is designed to recover communication in short time of 50 ms or less equivalent to that in the SDH (Synchronous Digital Hierarchy) or SONET (Synchronous Optical Network) on precondition that it is employed for a trunk communication system in which traffic of large volume flows like an urban network. Consequently, a high-reliable communication system can be constructed.
As a conventional technique for improving the reliability of an interlink connecting two RPR networks, there is link aggregation (herein below, described as “LAG”) disclosed in non-patent document 2. The non-patent document 2 is also a standardized document issued by IEEE.
The LAG is a technique of virtualizing a plurality of physical ports as a single logical port. In other words, it is a technique of virtualizing a plurality of links as a single logical link. By applying the LAG, in a normal state where no failure occurs, traffic is transmitted so as to be spread in the plurality of physical links belonging to the logical link. In such a manner, the communication band of the links can be increased to the sum of the communication bands of the physical links belonging to the logical link at the maximum. In an abnormal state where a physical link belonging to the logical link is disconnected, the communication can be continued by transferring the frames using only the other normal physical links in which no failure occurs.
FIG. 20 is an explanatory diagram showing an example of applying LAG to connection of RPR networks. In a communication system shown in FIG. 20, an RPR network 805 and an RPR network 905 are connected to each other. In the example shown in FIG. 20, each of an RPR node 800 belonging to the RPR network 805 and an RPR node 900 belonging to the RPR network 905 has ports P1 to P3 and, in addition, a port P4. The LAG is set in the ports P3 and P4 of the RPR node 800. Similarly, the LAG is set also in the ports P3 and P4 of the RPR node 900. The port P3 of the RPR node 800 and the port P4 of the PRP node 900 are connected via an interlink 491. The port P4 of the RPR node 800 and the port P3 of the RPR node 900 are connected via an interlink 492. With such a configuration, in the example shown in FIG. 20, the reliability of the connection between the RPR networks 805 and 905 is improved.
A technique of increasing the reliability of connection between two RPR networks is disclosed in patent document 1. FIG. 21 is an explanatory diagram showing the configuration of a network described in the patent document 1. In the network described in the patent document 1, as shown in FIG. 21, the RPR nodes 800 and 900 are disposed so as to belong to both of the RPR networks 805 and 905. One of the RPR nodes 800 and 900 performs, as an active node, frame transfer between the RPR network 805 and the RPR network 905. The other node is set, as a spare node, not to perform the frame transfer between the RPR networks. In the case where the spare node detects trouble in the active node, the spare node shifts to an operation mode of transferring frames between the RPR networks. The data in forwarding databases in all of the RPR nodes other than the RPR nodes 800 and 900 is erased. As a result, traffic transferred via the active node is transferred via the spare node. Consequently, even if trouble occurs in the active node, the communication can be performed continuously.
Patent document 2 describes a communication network system in which two ring networks are connected to each other. In the communication network system described in the patent document 2, a plurality of inter-ring connection node devices in a ring network and a plurality of inter-ring connection node devices in other ring network are connected.
Patent document 3 describes a method of connecting networks using a representative protocol address.
Patent document 1: Japanese Patent Application Laid-Open (JP-A) No. 2003-258822 (paragraphs 0015 to 0085, FIG. 1)
Patent document 2: JP-A-2000-4248 (paragraphs 0073 to 0076)
Patent document 3: Japanese Patent Publication No. 3599476 (paragraph 0068)
Non-patent document 1: “IEEE Standards 802.17 Part 17: Resilient packet ring (RPR) access method & physical layer specifications”, “5. Architecture Overview”, IEEE (Institute of Electrical and Electronics Engineers, Inc., 2004, p. 27-54)
Non-patent document 2: “IEEE Std 802.3ad Amendment to Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications”, “43. Link Aggregation”, IEEE (Institute of Electrical and Electronics Engineers, Inc), 2000, p. 95-107